Fuel efficiency of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermal efficiency by using a smaller displacement when full torque is not required. The most common method today of implementing a variable displacement engine is to deactivate a group of cylinders substantially simultaneously. In this approach the intake and exhaust valves associated with the deactivated cylinders are kept closed and no fuel is injected when it is desired to skip a combustion event. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three displacements.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders.
U.S. Pat. No. 8,131,445 (which is incorporated herein by reference) teaches a continuously variable displacement engine using a skip-fire operational approach, which allows any fraction of the cylinders to be fired on average using individual cylinder deactivation. In a continuously variable displacement mode operated in skip-fire, the amount of torque delivered generally depends heavily on the firing fraction, or fraction of combustion events that are not skipped. In other skip fire approaches a particular firing pattern or firing fraction may be selected from a set of available firing patterns or fractions.
In order to operate with skip fire control it is desirable to control the intake and exhaust valves in a more complex manner than if the cylinders are always activated. Specifically the intake and/or exhaust valves need to remain closed during a skipped working cycle to minimize pumping losses. This contrasts with an engine operating on all cylinders, where the intake and exhaust valves open and close on every working cycle. For cam operated valves a method to deactivate a valve is to incorporate a solenoid controlling a collapsible valve lifter into the valve train. To activate the valve the lifter remains at its full extension and to deactivate the valve the lifter is collapsed. Other mechanisms exist to deactivate valves in engines with cam operated valves. Engines with electronic valve actuation generally have more flexibility in the valve opening and closing because the valve motion is not constrained by rotation of a camshaft.
If cylinder deactivation occurs after a combustion event but prior to an exhaust event, all of the exhaust remains in the cylinder during the duration of deactivation. This condition may be referred to as the cylinder having a high pressure exhaust spring (HPES) in the cylinder. If instead, the cylinder deactivation occurs after the exhaust valve has opened but before the intake valve is opened, only a small residual charge remains in the cylinder. This condition may be referred to as the cylinder having a low pressure exhaust spring (LPES).
A potential problem with skip fire control is that if for some reason the exhaust gases associated with a cylinder firing have not been vented from the cylinder attempting to open the intake valve may damage the valve, push rod, lifter or any component in the valve train because of the high pressure contained in the cylinder. It is desirable if a determination of whether a cylinder has vented can be made prior to activation of the intake valve.